Natural gas generally refers to rarefied or gaseous hydrocarbons (comprised of methane and light hydrocarbons such as ethane, propane, butane, and the like) which are found in the earth. Non-combustible gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, “Mineral Terms-Some Problems in Their Use and Definition,” Rocky Mt. Min. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
It is common practice to cryogenically liquefy natural gas so as to produce a liquefied natural gas product (“LNG”) for more convenient storage and transport. A fundamental reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers at low or even atmospheric pressure. Liquefaction of natural gas is of even greater importance in enabling the transport of gas from a supply source to market where the source and market are separated by great distances and pipeline transport is not practical or economically feasible. In some cases the method of transport is by ocean going vessels. It is uneconomical to transport gaseous materials by ship unless the gaseous materials are highly compressed. Even then the transport would not be economical because of the necessity of providing containers of suitable strength and capacity.
In order to store and transport natural gas in the liquid state, the natural gas is typically cooled to −240° F. (−151° C.) to −260° F. (−162° C.) where it may exist as a liquid at near atmospheric pressure. Many LNG liquefaction plants utilize a mechanical refrigeration cycle for the cooling of the inlet gas stream, such as of the cascaded or mixed refrigerant types, as is generally disclosed in U.S. Pat. No. 3,548,606, the teachings of which are incorporated herein by reference. Various other methods and/or systems exist for liquefying natural gas whereby the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages, and cooling the gas to successively lower temperatures until liquefaction is achieved. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, nitrogen and methane, or mixtures thereof, in a closed loop or open loop configuration. The refrigerants can be arranged in a cascaded manner, in order of diminishing refrigerant boiling point. For example, processes for preparation of LNG generally are disclosed in U.S. Pat. Nos. 4,445,917; 5,537,827; 6,023,942; 6,041,619; 6,062,041; 6,248,794, and UK Patent Application GB 2,357,140 A. The teachings of all of the foregoing patents are incorporated herein by reference in their entirety.
Additionally, the liquefied natural gas can be expanded to atmospheric pressure by passing the liquefied gas through one or more expansion stages. During the course of the expansion, the gas is further cooled to a suitable storage or transport temperature and is pressure reduced to approximately atmospheric pressure. In this expansion to atmospheric pressure, significant volumes of natural gas may be flashed. The flashed vapors may be collected from the expansion stages and recycled or burned to generate power for the liquid natural gas manufacturing facility.
The cascaded refrigeration cycle type plants are typically relatively expensive to build and operate, and the mixed refrigerant cycle plants also can require close attention of stream compositions during operation. Refrigeration equipment is particularly expensive because of the low temperature metallurgy requirements of the components. However, liquefaction of natural gas is an increasingly important and widely-practiced technology to convert the gas to a form which can be transported and stored readily and economically. The costs and energy expended to liquefy the gas must be minimized to yield a cost-effective means of producing and transporting the gas from the gas field to the end user. Process technology which reduces the cost of liquefaction in turn reduces the cost of the gas product to the end user.
Process cycles for liquefaction of natural gas historically have utilized isentropic expansion valves, or Joule Thomson (J-T) valves, to produce refrigeration required to liquefy the gas. Typical process cycles utilizing expansion valves for this purpose are described for example in U.S. Pat. Nos. 3,763,658, 4,065,276, 4,404,008, 4,445,916, 4,445,917, and 4,504,296.
The work of expansion which is produced when process fluids flow through such valves is essentially lost. In order to recover at least a portion of the work produced by the expansion of these process fluids, expansion machines such as reciprocating expanders or turboexpanders can be utilized. For example, U.S. Pat. Nos. 4,445,916; 4,970,867; and 5,755,114 describe use of turboexpanders in connection with the production of LNG.
The term “expander” or “expander/compression device” as used herein generally is in reference to such turborexpanders or reciprocating expanders. In the field of natural gas liquefaction, the term “expander” is usually used to denote a turboexpander, and is so used in the present disclosure.
Applicants are unaware of any previous attempts to utilize the excess pressure of a methane-rich gas feed stream, such as a natural gas stream, as a source of refrigeration for a LNG process, such as to provide compression for a refrigeration cycle used to pre-cool the natural gas before it is directed to a liquefaction zone, or compression for one or more refrigeration cycles used to liquefy the natural gas in the liquefaction zone. While most liquefaction processes utilize a methane-rich feed which is typically delivered at a pressure of 650 psig (44.8 barg) to 1000 psig (69.0 barg), there are many instances where the supplied natural gas may be available at higher pressures, such as from about 1000 psig (69.0 barg) and to as high as 2500 psig (172.4 barg) or greater. This gas may be produced at such pressures from an underground geological formation; or it may be compressed to such pressure after it is produced for any number of reasons associated with the requirements of the production field; or it may be compressed due to the requirements of local pipelines or gas transmission systems adjacent to the production field. Use of such a preliminary step prior to liquefaction could result in a liquefaction plant that is less expensive to build and/or operate, and/or allow for a greater amount of LNG production for a given plant design. Alternatively, the excess pressure can be converted into mechanical work that may used to generate electrical power which could also yield a more efficient process.
As can be seen, it would be desirable to utilize the excess energy resident within such available gas streams in a manner which results in a more efficient and/or potentially less expensive LNG liquefaction process.